Introduction

Meningiomas of the third ventricle represent a distinct and rare anatomical subtype of meningiomas. They are not dural based and are thought to arise from arachnoid cap cells within the velum interpositum in the roof of the third ventricle. Third ventricle meningiomas may be further segregated by location into anterior and posterior third ventricular, with pineal region meningiomas included in the posterior group. Posterior third ventricle meningiomas were called posterior tumors of the velum by Cushing and Eisenhardt in their classic text.1 This anatomical segregation is useful both as a way to understand the divergent clinical presentations of these two categories of tumors and also in choosing among the various surgical approaches for access to the lesion. It is worth noting that meningiomas arising from the base of the third ventricle have been described.2 However, they are extremely rare and probably do not arise from the velum interpositum.

Before the advent of more sophisticated neuroimaging technology, several authors grouped falcotentorial meningiomas of the pineal region along with posterior third ventricle and pineal region meningiomas. However, it is now generally appreciated that these dural-based falcotentorial tumors are not true meningiomas of the third ventricle because they do not arise from the velum inter-positum. Following the suggestion of Stein,3 we prefer to limit our definition of posterior third ventricle and pineal region meningiomas to only those that arise from the velum interpositum.

Third ventricle meningiomas present a variety of diagnostic and therapeutic challenges for the treating neuro-surgeon. Acute presentations secondary to obstruction within the ventricular system, atypical imaging features, and technical hurdles that make stereotactic biopsy of targets in the third ventricle and pineal region challenging render third ventricle meningiomas among the most difficult to manage. The development of modern neuro-imaging and microneurosurgical techniques facilitates definitive surgical treatment, with a goal of gross total resection and low attendant morbidity. This chapter reviews the epidemiology, structural etiology, surgical anatomy, clinical and imaging features, and surgical approaches for resection of third ventricle meningiomas.

Epidemiology

Meningiomas of the third ventricle represent 0.15 to 0.18% of intracranial tumors and 6 to 15% of intraventricular meningiomas.4–6 Although the exact number remains in question, very few cases of third ventricular meningiomas have been reported.2,7–10 Indeed, it is likely that only a subset of reported cases arise from the velum interpositum and are thus, by definition, of third ventricular origin. A literature review by Renfro et al9 revealed 47 cases of third ventricular meningiomas since 1913. Of these, only 29 cases were truly thought to originate within the third ventricle based on their anatomical isolation to this region. Lozier and Bruce2 found 27 histologically verified velum interpositum meningiomas of the posterior third ventricle and pineal region, only 17 of which had sufficient clinical information for analysis. Although extraventricular meningiomas primarily present in adults, with a higher incidence in women, meningiomas of the third ventricle may not strictly follow this trend. Third ventricle meningiomas may occur slightly more frequently in men and are seen relatively frequently in children. Renfro et al9 found 12 reports of pediatric anterior third ventricle meningiomas among 29 cases reviewed from the literature. Within this pediatric cohort there was a slight male preponderance, with eight reports in boys and five in girls. This is in comparison with an earlier study by Cabezudo et al4 that did not reveal a sex predilection. Similarly Lozier and Bruce2 found four cases of pediatric velum interpositum meningiomas among their 17 reviewed cases. They too observed an increased incidence in males (11:6).

Developmental Considerations

The neural elements forming the structural framework for the roof of the third ventricle (see section on surgical anatomy) are lined by a double layer of tela choroidea called the velum interpositum. It can be thought of as a soft tissue partition between the diencephalon and the telencephalon.11 Embryologically, the third ventricle roof is formed by a single layer of tela choroidea.12 Formation of the velum interpositum begins as the fornix and corpus callosum extend posteriorly, causing the attached tela choroidea to reflect upon itself as it is pulled along. Anteriorly, these two layers become closely apposed, forming a potential space. Posteriorly, the layers separate with the inferior (ventral) layer coursing along the posterior wall of the third ventricle while the superior (dorsal) layer becomes adherent to the fornices and splenium of the corpus callosum. The potential space between this double layer may separate to form a velum interpositum cistern or cavum velum interpositum13 that directly communicates with the quadrigeminal cistern.

Surgical Anatomy

Understanding the anatomy of the third ventricle and its relation to surrounding structures allows for an appreciation of the structural basis for third ventricle meningioma development, which helps guide the planning and implementation of appropriate surgical approaches. The midline, slitlike, ependyma-lined third ventricle has a superior and an inferior part delineated by the hypothalamic sulcus. Meningiomas of the third ventricle are generally limited to the superior part between the paired thalami. The anatomy of the third ventricle is commonly described in terms of the structures forming its roof, anterior wall, floor, posterior wall, and lateral walls. The structural relations of the third ventricle are likewise delineated within this schema. A detailed description of the surgical anatomy of the third ventricle can be found elsewhere14; however, a concise review is presented here.

Structural Considerations

Meningiomas of the third ventricle arise from arachnoid cap cells within the velum interpositum and originate from either the ventral or dorsal layer of the tela choroidea. This distinction has great significance for posterior third ventricle and pineal region meningiomas. The anatomical constraints provided by the structures forming the third ventricle roof make this distinction less important for anterior third ventricle meningiomas. Meningiomas arising posteriorly from the ventral tela choroidea displace the internal cerebral veins dorsally and grow caudally toward the floor of the third ventricle. Those arising from the dorsal layer of the tela choroidea displace the internal cerebral veins ventrally and bow into the roof of the third ventricle. Additionally, these meningiomas may lie predominantly in the pineal region (quadrigeminal cistern) and are thought to arise from either the posterior velum along its course on the inferior surface of the splenium of the corpus callosum or along the posterior tenia fornicis.2,12

Clinical Presentation

The clinical presentation of third ventricle meningiomas is primarily due to a combination of cerebrospinal fluid (CSF) obstruction and compression of surrounding structures ( Table 33.1 ). Symptoms attributable to intracranial hypertension due to obstructive hydrocephalus are very common. Headache, cognitive disturbance, impaired gait, and visual symptoms are frequently seen at presentation. Transient symptoms presumably due to intermittent obstruction of the foramen of Monro have been reported for anterior lesions.4 Memory deficits and endocrine abnormalities are also more frequent, with anterior lesions likely secondary to forniceal and hypothalamic involvement, respectively.9 Posterior third ventricle and pineal region masses can present with extraocular movement disturbance and pupillary abnormalities from cranial nerve dysfunction and the Parinaud syndrome as a result of dorsal midbrain compression.

Imaging Characteristics

Meningiomas of the third ventricle may or may not demonstrate the typical features of dural-based meningiomas. Computed tomographic (CT) scans generally demonstrate a well-circumscribed iso- or hyperdense lesion with homogeneous contrast enhancement. As with dural-based meningiomas, calcifications may be evident. On magnetic resonance imaging (MRI), third ventricle meningiomas typically appear hypo- to isointense on T1 sequences, hyperintense on T2 sequences, with strong homogeneous postgadolinium enhancement ( Fig. 33.1 ). Because CSF also demonstrates strong T2 signal characteristics, fluid-attenuated inversion recovery (FLAIR) imaging is often useful for delineating ventricular masses. Tekkök et al15 reported a case of a third ventricle meningioma with minimal signal on T1 sequences and no contrast enhancement, but with easily detectable hyper-intensity on FLAIR imaging. Weak, heterogeneous postgadolinium enhancement has also been reported.2

Hydrocephalus with dilation isolated to the lateral ventricles is commonly seen with anterior third ventricle meningiomas due to foramen of Monro obstruction. Tectal plate compression causing aqueductal stenosis and triventricular dilation is seen with posterior third ventricle and pineal region meningiomas ( Fig. 33.2 ).

Contrast MRI sequences with gadolinium and magnetic resonance venography (MRV) may be useful in delineating the relationship of the tumor to the deep venous system, an important consideration for surgical planning. Whereas meningiomas arising from the dorsal layer of the tela choroidea displace the internal cerebral veins ventrally, those arising from the ventral layer of the tela choroidea displace the internal cerebral veins dorsally. Additionally, MRV supplies information regarding paten-cy of the deep venous system, thus aiding in assessment of the risk of inadvertent damage to these veins.2

Cerebral angiography may be useful for further delineating the vascular supply of third ventricle meningiomas. Variable degrees of tumor blush may be observed. Additionally, angiography will aid in the identification of meningiomas suitable for preoperative embolization. However, in most cases embolization will not be possible because velum interpositum meningiomas are generally supplied by the medial posterior choroidal arteries (PChAs), which are not routinely embolized with impunity. On the other hand, Sagoh et al demonstrated the potential utility of preoperative embolization of velum interpositum meningiomas that have feeders from external carotid artery branches.16

Opposite displacement of the plexal segment of the medial PChAs and the internal cerebral veins are thought to be characteristic of tumors originating within the velum interpositum.12 Although this finding is useful if present, it is important to note that meningiomas of the third ventricle may displace both vessels in the same direction.

The differential diagnosis for third ventricular lesions can be segregated anatomically into anterior third ventricle, posterior third ventricle, and pineal region lesions. Meningioma, astrocytoma, choroid glioma, craniopharyngioma, choroid plexus papilloma/carcinoma, and metastasis are the major entities in the anterior third ventricle that can share imaging features. In addition to these lesions, pineal parenchymal tumors and germ cell tumors must be considered in the differential for lesions in the posterior third ventricle and pineal region.